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End Sub
End Structure
■Note Nothing is a special value that you can assign to reference variables。 It means that the reference
variable does not point to a real object。 Nothing is a useful value to assign to reference variables in a constructor；
if you don’t have a meaningful object that they can point to at the outset。
To define a constructor； you define a method of type Sub called New（）。 And； in most cases；
you will use public scope。 The parameters of the constructor represent the three pieces of
information that are required to instantiate a valid state。 Within the constructor； the data
members are assigned the values of the parameters。
The defined constructor has parameters； which means that to instantiate Node； you need
to provide the three pieces of data。 Thus； to instantiate Node； you need to provide enough data
to make the node logical。 The original instantiation code would not pile； so to pile the
code； you need to modify the instantiation to the following：
Dim city as Node = New Node（〃Montreal〃； 0。0； 0。0）
The declaration of the node might reference incorrect data； but that is not the responsibility
of the intelligent data structure。 An analogy is that a word processor by itself is not responsible for
making sure that the text you write makes sense。 The role of the word processor is to give you
the ability to construct intelligent text。
Examining the Problem of Referencing Using Value Types
As you’ve learned； a value type is stored on the stack； and its contents are copied； not refer
enced。 When you are trying to build a tree structure with a value type； references that were
assigned are not updated with the correct information because values are copied。 This effect
can be demonstrated by going through a longer example of building a data structure of cities
that can be reached from another city。 To start off； consider the following declaration of all the
cities and their coordinates。
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92 CH AP T E R 4 ■ L E A R N IN G AB OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S
Dim montreal As Node = New Node（〃Montreal〃； 0； 0）
Dim newyork As Node = New Node（〃New York〃； 0； …3）
Dim miami As Node = New Node（〃Miami〃； …1； …11）
Dim toronto As Node = New Node（〃Toronto〃； …4； …1）
Dim houston As Node = New Node（〃Houston〃； …10； …9）
Dim losangeles As Node = New Node（〃Los Angeles〃； …17； …6）
Dim seattle As Node = New Node（〃Seattle〃； …16； …1）
This code creates several Node objects that represent all of the cities from Figure 4…7。 The
individual objects are cities without connections； and the next step is to connect one city to
another。 We need to allocate and assign the Connections data member。
Having initialized all of the individual cities； the next step is to cross…reference the destina
tions of each city。 In Visual Basic； the array requires some special understanding。 The Connections
data member is an empty array without any elements。 This is not a problem； as long as you
don’t attempt to reference or assign any of the elements。
You need to allocate space for the array so that you can store the individual cities。 One
solution is to reassign the array to another array that contains elements； as follows：
montreal。Connections = New Node（） ｛newyork； toronto； losangeles｝
In this example； the Connections data member is reassigned with the contents of another
array that has been allocated to contain the three cities； newyork； toronto； and losangeles。
Think of the array as a basket。 Using this approach； the original empty basket has been replaced
with a basket of three items。 These three items can be referenced using three indices （0 = newyork；
1 = toronto； 2 = losangeles）。 This solution creates a fixed…content basket。
Alternatively； you could create space in the basket and then manually fill the basket； as follows：
ReDim montreal。Connections（2）
montreal。Connections（0） = newyork
montreal。Connections（1） = toronto
montreal。Connections（2） = losangeles
The ReDim statement takes the array referenced by the data member Connections and real
locates it to a new size。 That size could be larger or smaller than the current size。 In the example；
the size of the array is 2； which does not mean two elements； but rather that it is an array sized
from the index 0 to 2。
The ReDim statement actually performs a redimension of an array。 The example demon
strates using it on an empty array； but you can also use it with an array that contains data； such
as to enlarge an array without losing the old data。 To keep the old data in a new array； you use
the Preserve keyword； as follows：
ReDim Preserve montreal。Connections（2）
montreal。Connections（0） = newyork
montreal。Connections（1） = toronto
montreal。Connections（2） = losangeles
However； be aware that if the ReDim statement causes an array to shrink； you can still lose
data—whatever elements are located above the array size are discarded。 To be safe； before
using ReDim； you might want to reference the array’s Length property to verify that you are not
shrinking the array and losing data。
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CH AP T E R 4 ■ L E A R N I N G A B OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S 93
Using either the fixed…content or ReDim app